More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown
Neighboring /51 subnets
- Expanded address
- Compressed address
- Network (expanded)
- Last address (expanded)
- Prefix mask
- Total addresses (exact)
- Reverse DNS (PTR)
- Host bits / network bits
Hextet breakdown
Quick facts for IPv6 /51
- Prefix mask: ffff:ffff:ffff:e000::
- Total addresses: 2⁷⁷
- Approx. count: 1.51 × 10²³
- /64 subnets: 2¹³
IPv6 /51 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ
Network / host bit split
- Network bits
- 51
- Host bits
- 77
- Prefix mask
- ffff:ffff:ffff:e000::
- Total addresses
- 2⁷⁷
- Approx. count
- 1.51 × 10²³
- /64 subnets
- 2¹³
- Addresses formula
- 2^77
- /64 relationship
- 2¹³ × /64 subnets
Overview
A /51 block fixes the first 51 bits of the address, leaving 77 host bits and 2⁷⁷ total addresses. It subdivides into 2¹³ /64 LAN subnets. A /51 is a site or subscriber delegation size — larger than a single /64 LAN but smaller than the classic /48 site block.
Common use cases
- Business site or campus delegation
- ISP customer prefix assignment
- Subdividing into /64 VLANs and LANs
Key facts
- A /51 fixes 51 network bits and leaves 77 host bits — 2⁷⁷ total addresses.
- In network design terms, /51 is typically a medium site or business delegation.
- You can subnet a /51 into 2¹³ /64 LANs.
Design guidance
A /51 is a practical site or campus delegation. Give each VLAN, Wi-Fi SSID, and security zone its own /64 — do not subnet below /64 on production LANs. With 2¹³ available /64s, you have ample room for guest networks, IoT, management, and lab segments.
Practical example
Suppose your ISP delegates 2001:db8:abcd:1200::/51 to your edge router. You could assign 2001:db8:abcd:0012::/64 to your main LAN, 2001:db8:abcd:0013::/64 to guest Wi-Fi, and 2001:db8:abcd:0014::/64 to IoT — using only 3 of the 2¹³ available /64 subnets.
Related RFCs and standards
- RFC 4291IPv6 Addressing Architecture
- RFC 6177IPv6 Address Assignment to End Sites
- RFC 3633IPv6 Prefix Options for DHCPv6 Prefix Delegation
Prefix sizing reference
Frequently asked questions
Common questions about IPv6 /51 blocks, prefix sizes, and use cases.